The present invention relates to a hysteresis clutch having an adjustably set stall torque that remains constant over a wide temperature range.
In many DC motor applications it is desirable to drive a mechanical load at a constant speed between predefined mechanical limits. In most cases it is possible to drive the load at a constant speed with relatively low torque when the path of the load is unobstructed. However, if significant resistance to the motion of the load is encountered, or when the load reaches its end of travel and encounters a mechanical stop, the torque on the output shaft of the motor rises dramatically. If the torque increases past a certain critical value (the motor stall torque) the motor will stop turning and the load will cease moving. In many cases it is desirable to maintain a constant torque applied against the load in order to firmly hold the load against mechanical stops after the load has reached the end of travel. A stable constant torque output, however, is difficult to achieve with a stalled DC motor. The stall torque of the motor varies as a function of motor temperature, and the excess current drawn by the stalled motor causes the motor temperature to increase rapidly. Thus, when the motor is stalled, the output torque may change significantly as the windings of the motor heat up.
Slip clutches of various designs are often provided to surmount this problem. A slip clutch is fitted to the output shaft of a DC motor between the motor and the load. A slip clutch comprises an input shaft adapted to be coupled to the output shaft of the motor, and an output shaft adapted to drive a load. The clutch interface acts to couple the driven clutch input shaft to the clutch output shaft to drive a load. Under normal conditions, the output shaft of the clutch rotates one-to-one with the input shaft. However, if the load on the output shaft exceeds a certain amount, the output shaft of the clutch breaks free of the input shaft, and the load side of the clutch stops moving while the motor side continues to rotate. If the torque on the output side of the clutch is reduced below the stall torque, the clutch will re-engage and the load will again begin to move. Ideally, while the output side of the clutch is stalled and the motor continues to turn, a uniform holding torque, or stall torque, will nonetheless be coupled to the output side of the clutch to maintain a constant force against the load.
An example of an application where such an arrangement is desirable is in the deployment of an antenna on a satellite or spacecraft. Typically, a DC motor will drive the antenna support hardware from a retracted position to an extended position. The extended position will generally be defined by mechanical stops, beyond which the antenna support hardware may not be extended further. As the antenna is deployed, the DC motor will generally drive the antenna support hardware at a constant speed and at constant torque. However, when the antenna reaches the point of full extension, the support hardware is driven against mechanical stops which prevent further extension of the antenna. At this point the torque on the output shaft of the motor rises substantially, and the motor stalls. Once the antenna reaches full extension, it is desirable to apply a constant torque against the support hardware to maintain the antenna in a fully deployed position.
In an application such as deploying a satellite antenna, the maintaining torque must be very precisely controlled. Ordinarily a slip clutch would be expected to be well suited for such a task, however, prior art slip clutches have not been able to meet the rigorous demands of space borne applications. On board a satellite, components are subjected to temperatures ranging from approximately -60.degree. C. to +150.degree. C. Maintaining a constant stall torque over such an extreme temperature range is beyond the capability of most prior art slip clutches. Friction clutches, for example, are especially prone to fluctuations in stall torque due to temperature changes as a result of the thermal expansion and contraction of mechanical parts.
Magnetic, or hysteresis type clutches are less prone to temperature induced variations in stall torque, however, prior art hysteresis slip clutches have been subject to other limitations that make them equally unsuitable for use on space vehicles, and in other demanding applications. For example, the internal inertia of the slip clutch is an important factor that must be considered in the design of an antenna deployment system for a space vehicle. Weight is a critical design criterion in satellite systems. If the drive mechanism for deploying the antenna is too heavy, and the spinning rotor of the DC motor or the output rotor of a slip clutch develops too much angular momentum, the size of the mechanical stops at the antenna support structure's end of travel must be increased in order to absorb the shock when the antenna support hardware is driven at full speed into the stops. In contrast, a smaller motor or slip clutch that develops less angular momentum requires less substantial stops and thereby provides a savings in the overall weight of the antenna deployment system.
In addition to supplying a slip clutch having the ability to maintain constant stall torque over a broad range of temperature extremes, it is also desirable to provide a slip clutch wherein the actual stall torque is adjustable. Manufacturing tolerances inevitably result in slip clutches--including those of identical design--having slightly different stall torques from one unit to the next. This result is unacceptable for those applications wherein a very precise pre-defined stall torque is required.
In light of the various shortcomings of prior art slip clutch designs, there is a need for an improved hysteresis clutch. The improved clutch must provide a mechanism for adjusting the stall torque of the output shaft of the clutch such that the clutch may be pre-set to stall when the torque on the output shaft of the clutch reaches a designated magnitude. Further, such an improved adjustable hysteresis clutch must maintain a constant stall torque on the output shaft of the clutch over a wide temperature range, for example in the range between -60.degree. C. to +150.degree. C. Finally, such an adjustable hysteresis clutch must have low inertia such that rotation of the clutch output may be arrested with relatively little effort.